The present application is based on Japanese patent application No. 2000-273738 (filed Sep. 8, 2000), the contents thereof are incorporated herein by reference.
The present invention relates to a parking brake unit for a work vehicle having a parking brake operated when the engine is stopped.
Conventionally, a negative parking brake has been provided in a work vehicle such as a wheeled hydraulic excavator or a wheel loader, in addition to a foot brake. Operation of this parking brake is released if release pressure is supplied from a hydraulic source after an engine has been started, and the parking brake is operated if supply of the release pressure is stopped when the engine is stopped.
In the case where an engine can not be started, such as because of vehicle failure etc., the vehicle shall be towed to a specified location. In this case, if the parking brake is kept operating, it becomes difficult to tow the vehicle away. For this reason, it is necessary to allow the parking brake to be released even if the engine is stopped. In the related art, the parking brake is released by operating a release lever or the like at outside of the operator""s cabin, which is provided close to the parking brake. However, it is troublesome to release the parking brake manually at outside of the operator""s cabin. Also, in the case where the release lever for the parking brake is provided at a place hidden from the view, operability is degraded.
The object of the present invention is to provide a parking brake unit for a work vehicle that releases a parking brake easily with an operation inside the operator""s cabin, even when the engine is stopped.
In order to achieve the above described object, a parking brake unit for a work vehicle comprises a hydraulic source; a negative parking brake provided a lower portion of the vehicle, that releases brake operation in response to brake release pressure; an operating member that outputs a release command for the parking brake in response to operation performed in an operator""s cabin; and a brake release means for releasing operation of the parking brake by leading the brake release pressure to the parking brake if the release command is output. And, the brake release means comprises a brake release accumulator that accumulates pressure oil from the hydraulic source, and a control valve that allows the pressure oil accumulated in the accumulator to act on the parking brake as the brake release pressure if the release command is output.
It is preferable that the parking brake is constituted by a transmission having a negative first clutch device and a negative second clutch device. The transmission sets a first gear ratio by allowing a propeller shaft to rotate if the first clutch device is released by control pressure for clutch release, and sets a second gear ratio smaller than the first gear ratio by allowing the propeller shaft to rotate if the second clutch device is released by the control pressure for clutch release. It is preferable that the control valve allows the pressure oil from the accumulator to act on the second clutch device as the brake release pressure if the release command is output. The control valve may allow the pressure oil from the accumulator to act on the first clutch device as the brake release pressure if the release command is output. The control valve may also allow the pressure oil from the accumulator to act on one of the first clutch device and the second clutch device as the brake release pressure if the release command is output.
It is also possible to further comprise a transmission control means, having a gear ratio control valve that controls flow of the control pressure for clutch release for acting on the first clutch device and the second clutch device, and a sudden operation prevention accumulator that prevents sudden operation of the parking brake, and the brake release means may be provided between the parking brake and the transmission control means.
It is desirable that the brake release means further comprises a prevention means for preventing the pressure oil accumulated in the accumulator flowing out to the hydraulic power side.
It is possible to further comprise a brake valve that generates brake operating pressure in correspondence with operation of a brake pedal; a foot brake unit that operates in response to the brake operating pressure; and a foot brake accumulator that accumulates specified pressure oil in order to generate the brake operating pressure in accordance with operation of the brake pedal when an engine is stopped. The foot brake accumulator may also function as the brake release accumulator.
In order to achieve the above described object, a parking brake unit for a work vehicle comprises a hydraulic source; a negative parking brake provided a lower portion of the vehicle, that releases brake operation in response to brake release pressure; an operating member that outputs a release command for the parking brake in response to operation performed in an operator""s cabin; and a brake release means for releasing operation of the parking brake by leading the brake release pressure to the parking brake if the release command is output. The parking brake is constituted of a transmission having a negative first clutch device and a negative second clutch device. The transmission sets a first gear ratio by allowing a propeller shaft to rotate if the first clutch device is released by control pressure for clutch release, and sets a second gear ratio smaller than the first gear ratio by allowing the propeller shaft to rotate if the second clutch device is released by the control pressure for clutch release. It is preferable that the brake release means comprises a brake release accumulator that accumulates pressure oil from the hydraulic source, and a control valve that allows the pressure oil accumulated in the accumulator to be guided to the second clutch device as the brake release pressure if the release command is output.
As described above, if the release command is output in response to an operation performed in the operator""s cabin, the brake release means allows the oil accumulated in the accumulator to flow, and leads the brake release pressure to the parking brake. Therefore, even if the engine is stopped, it is possible to release the parking brake through the operation in the operator""s cabin.
If the brake release command is output, pressure oil acts on the first clutch device or the second clutch device so that a locked state of the transmission is released. In this manner, it is not necessary to provide the parking brake separately. Also, if the gear ratio of the transmission is set to a second gear ratio that is smaller than a first gear ratio set by the first clutch device by guiding the pressure oil to the second clutch device when the brake release command is output, a towing operation can be performed easily because drive torque of the motor at the time of parking brake release is small.
The brake release means is provided between the parking brake and the transmission control means. As a result, since the pressure oil does not flow from the brake release means to the transmission control means, it is possible to minimize the brake release accumulator.
Also, by using the pressure oil from the foot brake accumulator as the release pressure for the parking brake unit, it is possible to reduce the number of components.